1. Field of the Invention
The present invention relates to a time division highway switch in general, and more particularly to a photonic frequency routing type time division highway switch which can be used as a self-routing switch in a photonic ATM switching system.
2. Description of the Background Art
A conventionally known M.times.M photonic frequency routing type time division highway switch has a typical configuration as shown in FIG. 1, which comprises: M input highways 1-1-1 to 1-1-M connected to an input side of a M.times.M star coupler 1-3 through M frequency convertors 1-2-1 to 1-2-M, and M output highways 1-5-1 to 1-5-M connected to an output side of the star coupler 1-3 through M fixed filters 1-4-1 to 1-4-M.
In this configuration, each frequency convertor assigns a frequency channel corresponding to a desired output highway for time divisional optical signals at each timeslot on each input highway connected with it. The optical signals on the input highways 1-1-1 to 1-1-M are then optically coupled together by the star coupler 1-3, and the coupled optical signals are equally split among the output highways 1-5-1 to 1-5-M. Each fixed filter connected with each output highway takes out only the optical signal of the frequency channel corresponding to each output highway connected with it out of the supplied coupled optical signals and outputs it to the output highway connected with it. In this manner, the switching operation is realized in this conventional photonic frequency routing type time division highway switch by assigning the frequency channel at the frequency convertor.
Now, such a conventional photonic frequency routing type time division highway switch has been associated with the following problems.
(1) In a case it is necessary to connect two optical signals on different input highways to the same output highway simultaneously, the same frequency channel is going to be assigned to both of these optical signals. Consequently, these two optical signals are going to conflict with each other at the star coupler 1-3, i.e., a routing of more than one optical signals with the same frequency is going to be requested at the same timeslot, so that there is a need to carry out the conflict control at the input side in advance in order to reject such conflicting requests. PA1 (2) The optical signals inputted into the star coupler 1-3 are going to be equally split and distributed among all the output highways 1-5-1 to 1-5-M connected to the star coupler 1-3. For this reason, the optical power of each input optical signal is going to be attenuated to 1/M on each output highway, where M is a total number of the output highways 1-5-1 to 1-5-M. In other words, the conventional photonic frequency routing type time division highway switch inherently incorporates the splitting loss of the optical signal power.
On the other hand, a conventionally known photonic buffer memory has a typical configuration as shown in FIG. 2, which comprises M opto-electronic convertors 2-1-1 to 2-1-M connected with M input ports I-1 to I-M, an electronic memory 2-2 formed by elements such as shift registers connected with the opto-electronic convertors 2-1-1 to 2-1-M, and an electro-optic convertor 2-3 connected with the electronic memory 2-2 and an output port O.
In this configuration, the optical packet signals inputted from the input ports I-1 to I-M are converted into electrical signals by the opto-electronic convertors 2-1-1 to 2-1-M, and stored in the electronic memory 2-2 in forms of the electrical signals. Then, the stored signals are converted into optical signals by the electro-optic convertor 2-3, and outputted to the output port O.
Now, in such a conventional photonic buffer memory, the optical packet signals must be converted into the electrical signals once, so that the bandwidth of the optical packet signals that can be handled is going to be limited by the characteristics of the convertors and the memory, and it has been difficult to handle the high speed optical signals such as the optical pulses with a pulse width in an order of pico-seconds.